Fix README following cjd's review + small things

* For the "Why?" section, I really tried to avoid thorny morality issues.
  Starting discussions about what "unpopular content" should be censored opens
  up a can of worms for other problems. The Tor faces the same issue (i.e. by
  supporting freedom) and they defend their position with the simple goal of
  defending the "right to read", which is commendable.

  I realized that at the heart of cjdns is protection of private
  communications, so that's the angle I've gone for, while trying to avoid
  introductions to morality discussions.

* Added a link to the buildbot page cjd provided

* Changed "thanks" to "cjdns developers", but I think the line should just be
  removed. Not many other projects thank readers for enduring their README :)
  It's a bit like saying "thank you" before receiving any applause.

Changes not requested by cjdns: (is this bad review process etiquette?
if so, let me know)

* Removed the first paragraph in "How does routing work" because it doesn't
  really help much with understanding how the existing Internet works (nor with
  understanding cjdns routing beyond what the second paragraph explains?)

* Changed the "protect your conf file" command to `chmod 600`, because the user
  will want to be able to write to it too (and that does no harm to security).